The invention relates to a knee-joint prosthesis with a hydraulic damping cylinder as set forth in the preamble of claim 1. The damping properties of a hydraulic cylinder for use in a prosthetic knee can be designed to be controllable in order to take account of the different resistance demands in the swing phase and the stance phase. One possibility of adaptation lies in electronic control via electrical actuating members which either change the current conditions or affect the viscosity properties of the fluid. A change in current conditions can be effected via a regulator valve, and a change in viscosity properties can be effected by acting, for example, on magnetorheological fluids.
As regards changing the viscosity properties of a fluid, it is known from the prior art for an actuating member, as a function of the phase of movement, to change the viscosity of the fluid by application of a magnetic field, with a high resistance and consequently a high viscosity being necessary in particular in the stance phase. Since the stance phase generally lasts longer than the swing phase, a relatively high current is always needed to safely permit standing.
Starting from this prior art, the object of the invention is to make available a knee-joint prosthesis which can be controlled with lower power consumption and which additionally guarantees increased safety in the event of the control system failing.
According to the invention, this object is achieved by a knee-joint prosthesis with the features of claim 1.
Advantageous embodiments and developments of the invention are set out in the dependent claims.
By means of an actuating member which generates a permanent force field and acts on the hydraulic fluid with an existing force field which is weakened or strengthened by the electronically controlled force field, only a low current need be applied for the swing phase in order to weaken the existing force field via the electronically controlled force field or to increase stance safety by means of an electronically controlled force field correspondingly differently oriented. In the event of a failure of the electronic control system, the stance stability of the prosthesis wearer is thus further guaranteed and a movement possibility, albeit a limited one, remains. In addition, the maintenance intervals for the knee-joint prosthesis can be extended, since overall there is a lower power consumption.
One embodiment of the invention involves a premagnetized actuating member which acts on a magnetorheological hydraulic fluid with a magnetic field.
In order to provide a constant magnetic field, the actuating member advantageously has a permanent magnet whose magnetic field is strengthened or weakened via a suitably arranged magnet coil to which voltage is applied.
As an alternative to this, provision is made for the force field to be an electrical field and for the hydraulic fluid to be an electrorheological fluid, the actuating member having electrodes made of an electret.
A particularly compact construction of the knee-joint prosthesis and of the damping device is obtained if the actuating member is located inside a piston or is integrated in the latter, which piston moves in the damping cylinder so that no additional structural space is needed to accommodate the actuating member. In a development of the magnetorheological variant, provision is made for the damping cylinder to be made of a ferromagnetic material in order to achieve a higher degree of integration.
In a further development, provision is made for a passage for the movement of the hydraulic fluid to be present between the inside wall of the cylinder and the hydraulic piston, so that the cylinder wall forms part of the magnetic circuit or the field excitation closed by the piston, the embedded permanent magnet or electrets via the passage and the cylinder. The passage is advantageously designed as an annular gap so that leakage takes place around the piston.
As an alternative to this, a passage for the movement of the magnetorheological or electrorheological fluid is formed inside the piston, so that the magnetic circuit or the field closes inside the piston and there changes the viscosity of the fluid flowing through. In this embodiment, it is not necessary for the cylinder to be made of a ferromagnetic material. The passage can be designed as a bore, channel or annular gap with diamagnetic bridges for magnetorheological fluids, in which case a plurality of bores or channels can also be formed inside the piston or at the piston edge.
An advantageous form of energy supply to the actuating member consists of a feed line through the piston rod, by which means the energy can be fed in a very stable manner and immediately to the magnet coil or the electrodes inside the piston.
In another embodiment of the invention, it is provided that the cylinder volumes which can be changed by the hydraulic piston are connected via a connecting line or leakage line and the actuating member acts on the magnetorheological or electrorheological fluid in the leakage line or connecting line with a field or changes the existing field in the line. Such an arrangement involves a relatively low outlay in construction terms and can be realized with conventional hydraulic means and field-generating elements.
The invention is explained in more detail below on the basis of illustrative embodiments. Identical reference numbers in different figures designate identical structural elements.